Such systems are known in the prior art. For instance, U.S. Pat. No. 6,104,947 relates to a method for determining exertion levels in fitness or athletic training and for determining the stress caused by the training by means of heartbeat rate measurement. Determining of the heartbeat rate is based on an ECG signal. Variations therein are accurately monitored, for example by means of standard deviation. The most important cause of the variation is respiratory arrhythmia. Based on the variations, a corresponding target heartbeat rate range is given or alternatively an alarm is given if a previously set target heartbeat rate range is exceeded or fallen short of Thus, by real-time monitoring of the heartbeat rate variation, the training of the individual person can be optimized.